Neurodegeneration is progressive loss of structure or function of neurons, including death of neurons.
Neurodegenerative diseases are the subset of neurological disorders that include neuron pathologies, but which exclude diseases of the nervous system due to cancer, edema, hemorrhage, trauma, poisoning, hypoxia, etc. Multiple sclerosis does not qualify as a neurodegenerative disease because it affects the myelin sheath rather than neurons.
Diseases involving neurodegeneration include Alzheimer's disease, Parkinson's disease, Huntington disease and Amyotrophic lateral sclerosis. The most consistent risk factor for developing neurodegenerative disease is aging.